Water-based pigment dispersion for ink-jet recording, ink composition for ink-jet recording and method of producing the same

ABSTRACT

Disclosed are an aqueous pigment dispersion for inkjet ink, a method for producing the same, and an ink composition for inkjet recording which mainly contains the aqueous pigment dispersion for inkjet ink. The method for producing the aqueous pigment dispersion for inkjet ink is characterized by comprising a kneading step wherein a mixture including a styrene resin, a quinacridone pigment, a phthalimidomethylated quinacridone compound, an alkali metal hydroxide and a wetting agent is kneaded for producing a solid colored kneaded material, and a dispersing step wherein the thus-obtained solid colored kneaded material is dispersed into an aqueous medium.

TECHNICAL FIELD

The present invention relates to a method of producing a water-basedpigment dispersion for red or magenta ink-jet ink and an ink compositionfor ink-jet recording, and to a water-based pigment dispersion forink-jet ink and an ink composition for ink-jet recording produced by themethod.

BACKGROUND ART

As a water-based ink for ink-jet recording, The pigmented ink has beendeveloped. The pigmented ink is expected to have excellent waterresistance and light fastness, but has problems such as change over timedue to aggregation or sedimentation of the pigment, and clogging of anozzle of print head. To solve these problems, a method of dispersing apigment in an aqueous medium using a resin as a dispersant is employed.As this method, there has been proposed a method of dispersing a resinwhich has an acid value in water in the presence of a basic component toobtain an aqueous solution, adding a pigment, sufficiently stirring themixture, and dispersing the pigment using a paint shaker or a sand mill(see Patent Document 1). However, since this method does not include theprocess of producing a colored kneaded mixture by a kneading prior todispersing process, long dispersing process time is required, causing aproblem such as low production efficiency. The resin used as adispersant is not selected enough in this method. Since the water-basedpigment dispersion thus obtained is insufficient in dispersionstability, it is difficult to stably disperse the pigment for a longperiod of time by using only the resin as the dispersant. Furthermore,it is impossible to realize good ejection properties of the ink, andgood image quality and durability of printed images. Thus, a water-basedpigmented ink composition for ink-jet recording having excellentlong-term storage stability has never been provided.

As for resin used as dispersant, a styrene-based resin has been studied.For example, there is known an example of a water-based pigmentdispersion for ink-jet ink in which a mixture of a humectant, sodiumhydroxide and carbon black is dispersed by using styrene-acrylic resin(styrene/acrylic acid/methacrylic acid=77/10/30, molecular mass: 7200and 8300) as the dispersant with a paint shaker (see Patent Document 2).According to the method of producing a water-based pigment dispersiondescribed in the above publication, it is made possible to produce awater-based ink for ink-jet recording having excellent dispersionstability in which carbon black has a fine particle size, and thewater-based ink is excellent in ink ejection properties. However, sincethe kneading process for producing a colored kneaded mixture is notincluded and an effective auxiliary dispersant is not used, thewater-based ink was insufficient in long-term dispersion stability whenused for a thermal jet type ink-jet printer. When this method is appliedto an azo pigment or quinacridone pigment, which is considered to havepoor dispersibility, good dispersibility is not attained as comparedwith carbon black.

Trials adding various pigment derivatives as auxiliary dispersants otherthan using the resin as the dispersant, were made so as to stablymaintain a dispersion state of the pigment for a long period.

For example, there is proposed a method of utilizing electrostaticrepulsion of a sulfonic acid group-containing pigment derivativeadsorbed onto the surface of a pigment using a pigment and a sulfonicacid group-containing pigment derivative (see Patent Document 3).According to this method, although dispersion stability of a colorant isimproved, printed matter obtained has high affinity with water becauseof the presence of a polar functional group on the surface of thecolorant. Furthermore, according to this method, dispersion stability isimproved only by the addition of the pigment derivative, and thus theremay arise a problem of poor water resistance.

To attain excellent dispersibility and dispersion stability of awater-based pigment dispersion or recording ink, which contains aquinacridone-based pigment as the colorant, there are proposed anencapsulated pigment dispersion wherein a quinacridone-based pigment anda dimethylaminomethylated quinacridone compound are used in combination(see Patent Document 4), and a water-based pigment dispersion containinga quinacridone-based pigment, a phthalimidomethylated quinacridone-basedcompound, a quinacridonesulfonic acid-based compound, and a anionicgroup-containing organic polymeric compound having a glass transitionpoint of −20 to 60° C. (see Patent Document 5). However, the magentawater-based pigment dispersion described in these publications has neverbeen studied regarding selection of the resin used as dispersant and thekneading method suited for the resin to be used, and thus it was notsufficient in dispersion level and dispersion stability.

It was difficult to produce a water-based pigment dispersion with gooddispersibility, containing a quinacridone pigment having poordispersibility, only by using the dispersant or auxiliary dispersant.

To improve dispersibility of the water-based pigment dispersion in theproduction process, a mixture of a resin and a pigment, or a mixture ofan aqueous resin solution containing a resin, water, a water-solubleorganic solvent, and a pigment is previously milled by rollers prior tothe dispersing step. In the method of milling using twin rollers, thereis employed a method of kneading the mixture to obtain solid chipscontaining a pigment, mainly adding water and a water-soluble organicsolvent to the solid chips, and dispersing them using a high-speed mixeror homogenizer to obtain a water-based pigment dispersion (see, forexample, Patent Documents 6 and 7).

To easily produce a resin solution, an organic amine compound is addedduring kneading and a specific pigment derivative is also used (see, forexample, Patent Document 8).

A water-based pigment dispersion is produced by kneading astyrene-acrylic resin having a weight-average molecular weight of 50000using twin rollers (see, for example, Patent Document 9).

By using the above method, the pigment is finely ground between rollersunder shear, and coating with the resin as the dispersant proceeds onthe surface of the finely ground pigment. To optimize kneadingconditions of the mixture of the resin and the pigment, kneading iscarried out after adding a solvent thereby to dissolve the resin, ormelting the entire mixture. Therefore, in the case of preparing awater-based pigment dispersion from the kneaded mixture, removal of thesolvent and crushing of the melted kneaded mixture are required and thedispersing time is prolonged, and thus a water-based pigment dispersionhaving good dispersion stability is not always obtained. For example,kneading using twin rollers is conducted in an open system and water andthe water-soluble organic solvent are vaporized during kneading and,finally, solid chips having high solid content are obtained. In thedispersing step, crushing and dispersing of the pigment in the form ofsolid chips must be conducted after adding water and the water-solubleorganic solvent. Since the pigment coated with the resin as thedispersant is crushed, the coating of the surface of the pigment maybecome insufficient.

Only by passing through the kneading step, the pigment in thewater-based pigment dispersion is finely ground and initialdispersibility is improved. However, the water-based pigment dispersionis insufficient in long-term dispersion stability under ahigh-temperature environment on the assumption that it is used for athermal jet type ink-jet printer.

To obtain a satisfactory aqueous dispersion of a quinacridone-basedpigment having poor dispersibility, it is important to use a methodcapable of effectively realizing coating of the surface of the pigmentwith the resin and adsorption of the auxiliary dispersant onto thesurface of the pigment by selecting the resin as the dispersant and anauxiliary dispersant.

However, there has never been found a method of producing an inkcomposition for ink-jet recording containing a quinacridone-basedpigment, capable of simultaneously realizing dispersibility, long-termdispersion stability, and ink ejection properties during image printingat a high level. Thus, it has been strongly required to develop an inkcomposition for ink-jet recording which is used to satisfactorilyperform thermal jet type ink-jet recording, and a method of producing awater-based pigment dispersion for ink-jet ink used to produce the aboveink composition.

-   Patent Document 1: Japanese Patent Application, First Publication    No. Hei 10-60332-   Patent Document 2: Japanese Patent Application, First Publication    No. 2002-256201-   Patent Document 3: Japanese Patent Application, First Publication    No. 2002-241638-   Patent Document 4: Japanese Patent Application, First Publication    No. Hei 9-151342-   Patent Document 5: Japanese Patent Application, First Publication    No. 2000-191974-   Patent Document 6: Japanese Patent Application, First Publication    No. Hei 6-157954-   Patent Document 7: Japanese Patent Application, First Publication    No. 2000-80299-   Patent Document 8: Japanese Patent Application, First Publication    No. 2001-81390-   Patent Document 9: Japanese Patent Application, First Publication    No. Hei 10-88042

An object of the present invention is to provide a red or magentawater-based ink composition for ink-jet recording, which is excellent indispersibility and dispersion stability and is less likely to cause anincrease in particle size even when allowed to stand at high temperaturefor a long period, and also which is less likely to cause an increase inviscosity and is excellent in printing stability, a water-based pigmentdispersion for ink-jet ink used to produce the same, and a method ofproducing the same.

Another object of the present invention is to provide a water-based inkcomposition for an ink-jet recording suited for a thermal ink-jetrecording system.

DISCLOSURE OF THE INVENTION

The present inventors have intensively researched about the aboveproblems and found that these problems can be solved by using thefollowing means.

Thus, the present invention provides a method of producing a water-basedpigment dispersion for ink-jet ink, which includes a kneading processfor kneading a mixture containing a styrene-based resin, aquinacridone-based pigment, a phthalimidomethylated quinacridone-basedcompound, an alkali metal hydroxide and a humectant to produce a coloredkneaded solid mixture, and a dispersing process for dispersing thecolored kneaded solid mixture in an aqueous medium, wherein thestyrene-based resin has 60% by mass or more of a styrene-based monomerunit based on all monomer components, a monomer unit containing anunsaturated aliphatic carboxylic acid having a radical polymerizabledouble bond, an acid value of 50 to 300 and a weight-average molecularweight of 7500 to 40000.

According to the method of the present invention, since a mixture of aquinacridone-based pigment, a phthalimidomethylated quinacridone-basedcompound, a styrene-based resin, a humectant and an alkali metalhydroxide is kneaded by a high shear force in the kneading step, thequinacridone-based pigment is crushed into fine particles and, at thesame time, the styrene-based resin provided with water dispersibility inthe presence of the alkali metal hydroxide is effectively adsorbed ontothe surface of the finely ground quinacridone-based pigment, and thusthe surface of the pigment is coated. Since the styrene-based resin usedin the present invention contains a high content, 60% by mass or more,of a styrene-based monomer unit, it shows good adsorption onto thehydrophobic surface of the pigment, and the encapsulation of the pigmentdue to the resin smoothly proceeds. Consequently, pigment particlesencapsulated efficiently in the kneading step are rapidly dispersed inthe aqueous medium in the dispersing step. At the same time, in thekneading step, the phthalimidomethylated quinacridone-based compound asthe auxiliary dispersant is used. In the kneading step under a highshear force, the phthalimidomethylated quinacridone-based compound issatisfactorily adsorbed onto the surface of the quinacridone-basedpigment, and dispersion stability of the water-based pigment dispersionis improved when the kneaded mixture is dispersed by a repulsive forcedue to steric hindrance. A water-based ink for ink-jet recording, whichcontains pigment particles having a very small particle size and isexcellent in dispersion stability, can be efficiently produced byfurther diluting the resulting water-based pigment dispersion with anaqueous medium and optionally adding various additives, and thus thepresent invention is very effective from industrial points of view.

The present invention also provides a water-based pigment dispersion forink-jet ink produced by the above method.

The present invention also provides an ink composition for ink-jetrecording including, as a main component, the water-based pigmentdispersion for ink-jet ink produced by the above method of the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

Now the respective components used in the kneading step and thedispersing step will be described and also the respective steps will bedescribed in more detail.

(1) Styrene-based Resin

The styrene-based resin used in the present invention is a resin whichhas (i) 60% by mass or more of a styrene-based monomer unit based on allmonomer components, (ii) a monomer unit containing an unsaturatedaliphatic carboxylic acid having a radical polymerizable double bond, anacid value of 50 to 300 and a weight-average molecular weight of 7500 to40000.

As used herein, the weight-average molecular weight is a value, asmeasured by a GPC (gel permeation chromatography) method, reduced interms of a molecular mass of polystyrene used as a certified referencesubstance.

The measurement was carried out by the following apparatus andconditions.

-   Delivery pump: manufactured by Shimadzu Corporation under the trade    name of “LC-9A”, System controller: manufactured by Shimadzu    Corporation under the trade name of “SIL-6B”, Auto-injector:    manufactured by Shimadzu Corporation under the trade name of    “SIL-6B”, Detector: manufactured by Shimadzu Corporation under the    trade name of “RID-6A”-   Data processing software: manufactured by System Instruments Co.,    Ltd., under the trade name of “Sic480II Data Station”-   Column: manufactured by Hitachi Chemical Co., Ltd. under the trade    name of “GL-R400 (Gard Column)”+“GL-R440”+“GL-R450”+“GL-R400M”,    Eluent: THF, Flow rate: 2 ml/min, Column temperature: 35° C.

Since the styrene-based resin used in the present invention contains alarge amount of the styrene-based monomer unit, it shows goodadsorptivity to the hydrophobic surface of pigment, and thus the surfaceof the pigment can be satisfactorily coated in the form of a capsule.

As the styrene-based monomer, known compounds can be used. For example,there can be used alkylstyrenes such as styrene, α-methylstyrene,β-methylstyrene, 2,4-dimethylstyrene, α-ethylstyrene, α-butylstyrene andα-hexylstyrene; halogenated styrenes such as 4-chlorostyrene,3-chlorostyrene and 3-bromostyrene; and 3-nitrostyrene, 4-methoxystyreneand vinyltoluene.

The content of the styrene-based monomer unit as a material of thestyrene-based resin is preferably from 60 to 90% by mass, andparticularly preferably from 70 to 90% by mass. When the content of thestyrene-based monomer is less than 60% by mass, affinity of thestyrene-based resin to the quinacridone-based pigment becomesinsufficient and thus dispersion stability of the water-based pigmentdispersion for ink-jet ink may be lowered. Also plain paper recordingproperties of the water-based ink for ink-jet recording compositionobtained from water-based pigment dispersion may be lowered and theimage recording density may be lowered and, furthermore, the waterresistance may be lowered. On the other hand, when the content of thestyrene-based monomer is within the above range, dispersibility of thestyrene-based resin in an aqueous medium can be improved anddispersibility and dispersion stability of the pigment in thewater-based pigment dispersion for ink-jet ink can be improved.Furthermore, printing stability is improved when used as the water-basedink composition for ink-jet recording.

As the unsaturated aliphatic carboxylic acid having a radicalpolymerizable double bond to be copolymerized with the styrene-basedmonomer, known compounds can be used. Examples thereof include acrylicacid, methacrylic acid, α-ethylacrylic acid, crotonic acid,α-methylcrotonic acid, α-ethylcrotonic acid, isocrotonic acid, maleicacid, fumaric acid, itaconic acid, citraconic acid, measaconic acid andglutaconic acid. Among these compounds, acrylic acid or methacrylic acidare preferably used, and the two are particularly preferably used incombination. By using acrylic acid in combination with methacrylic acid,copolymerizability on synthesis of the resin is improved and thusuniformity of the resin is improved. Consequently, there may be obtaineda pigment dispersion which is excellent in storage stability andcontains finely ground particles.

As the monomer of the styrene-based resin in the present invention,three kinds of monomers such as styrene-based monomer, acrylic acid andmethacrylic acid are preferably used. The total content thereof ispreferably 95% by mass or more based on all monomer components so as toenhance dispersibility and to increase a glass transition point.

As the styrene-based resin, for example, known monomers other than thestyrene-based monomer and the unsaturated aliphatic carboxylic acidhaving a radical polymerizable double bond can be used. Examples of themonomer include acrylate esters and methacylate esters, such as methylacrylate, n-propyl acrylate, isopropyl acrylate, sec-butyl acrylate,tert-butyl acrylate, 2-ethylbutyl acrylate, 1,3-dimethylbutyl acrylate,hexyl acrylate, 2-ethylhexyl acrylate, octyl acrylate, methylmethacrylate, ethyl methacrylate, n-butyl methacrylate, 2-methylbutylmethacrylate, pentyl methacrylate, heptyl methacrylate and nonylmethacrylate; acrylate ester derivatives and methacrylate esterderivatives, such as 3-ethoxypropyl acrylate, 3-ethoxybutyl acrylate,dimethylaminoethyl acrylate, hydroxyethyl acrylate, hydroxybutylacrylate, ethyl-α-(hydroxymethyl) acrylate, dimethylaminoethylmethacrylate, hydroxyethyl methacrylate and hydroxypropyl methacrylate;acrylic acid aryl esters and acrylic acid aralkyl esters, such as phenylacrylate, benzyl acrylate, phenylethyl acrylate and phenylethylmethacrylate; monoacrylate esters or monomethacrylate esters ofpolyhydric alcohol, such as diethylene glycol, triethylene glycol,polyethylene glycol, glycerin and bisphenol A; maleic acid dialkylesters such as dim ethyl maleate and diethyl maleate, and vinyl acetate.One or more kinds of these monomers can be added as a monomer component.

The weight-average molecular weight of the styrene-based resin used inthe present invention is within a range from 7500 to 40000, and morepreferably from 7500 to 30000. The weight-average molecular weight isparticularly preferably within a range from 10000 to 25000. When theweight-average molecular weight is less than 7500, initial dispersionand reduction in particle size of the quinacridone-based pigment can beeasily conducted. However, long-term storage stability of thewater-based pigment dispersion for ink-jet ink may be lowered andsedimentation due to aggregation of the pigment may occur. On the otherhand, when the weight-average molecular weight of the styrene-basedresin is more than 40000, the viscosity of the ink composition forink-jet recording prepared from the water-based pigment dispersion forink-jet ink containing the same increases and ejection stability of theink may be reduced especially in thermal jet type ink-jet recording.

The glass transition point of the styrene-based resin is preferably 90°C. or higher, and is more preferably within a range from 100° C. to 150°C. When the glass transition point is 90° C. or higher, thermostabilityof the ink composition is improved. Therefore, even when the water-basedink for ink-jet recording composition prepared from the water-basedpigment dispersion is used for thermal jet type ink-jet recording, theremay not arise a change in properties, which causes ink ejection failuredue to repeated heating, and thus it is preferred.

As a method of producing the styrene-based resin, a conventionalpolymerization method can be employed and examples thereof include amethod of polymerization reaction in the presence of a polymerizationcatalyst, for example, solution polymerization, suspensionpolymerization or bulk polymerization. Examples of the polymerizationcatalyst include 2,2′-azobis(2,4-dimethylvaleronitrile),2,2′-azobisisobutyronitrile, 1,1′-azobis(cyclohexane-1-carbonitrile),benzoyl peroxide, dibutyl peroxide and butylperoxy benzoate. The contentis preferably form 0.1 to 10.0% by mass based on the vinyl monomercomponent.

The styrene-based resin used in the present invention may be a randomcopolymer or a graft copolymer. The graft copolymer includes, forexample, a graft copolymer in which a copolymer of styrene and nonionicmonomer copolymerizable with styrene or polystyrene constitutes a stemor branch, and a copolymer of acrylic acid, methacrylic acid and theother monomer including styrene constitutes a branch or stem. Thestyrene-based resin may be a mixture of the graft copolymer and therandom copolymer.

(2) Quinacridon-based Pigment

As the quinacridone-based pigment in the present invention, anyconventionally known one can be used. Specific examples thereof includedimethylquinacridone-based pigments such as C.I. Pigment Red 122;dichloroquinacridone-based pigments such as C.I. Pigment Red 202 andC.I. Pigment Red 209; non-substituted quinacridones such as C.I. PigmentViolet 19; and mixtures or solid solutions of at least two kinds ofpigments selected from among these pigments. The pigment may be a drypigment in the form of powder, granule or bulk, or may be in the form ofa wet cake or slurry. Among the quinacridone-based pigments, C.I.Pigment Red 122 is preferable.

(3) Alkali Metal Hydroxide

As the alkali metal hydroxide in the present invention, for example,sodium hydroxide, potassium hydroxide and lithium hydroxide can be used.Among these, potassium hydroxide is preferably used. The amount of thealkali metal hydroxide is preferably 0.8 to 1.2 times the amountrequired to neutralize all carboxyl groups of the styrene-based resin.By neutralizing carboxyl groups in the styrene-based resin used in thepresent invention with the alkali metal hydroxide during kneading,excellent water dispersibility is attained.

(4) Auxiliary Dispersant

The phthalimidomethylated quinacridone-based compound used in thepresent invention can be synthesized, for example, by reactingnon-substituted quinacridone, dimethylquinacridone ordichloroquinacridone with phthalimide and aldehyde in concentratedsulfuric acid, and is preferably a compound represented by the followingformula (I):

wherein R and R′ each independently represents hydrogen, halogen, C1-C5alkyl or C1-C5 alkoxy, m represents 0, 1 or 2, and n represents 1 to 4.

n is more preferably from 1 to 2.

Specific examples of more preferred compounds among the compoundsrepresented by the formula (I) include compounds represented by thefollowing formula (III) having a group represented by the followingformula (II):

wherein m and n each independently represents 0, 1 or 2, provided that mand n are not simultaneously 0.

Most preferred compound used in the present invention is a compoundhaving 1 to 2 groups represented by the formula (II) per molecule onaverage. The compound is particularly preferably a compound having 1 to1.5 groups per molecule on average. When the number of the grouprepresented by the formula (II) is less than 1 per molecule on average,the effect on dispersion stability may not be exerted. On the otherhand, when the number of the group is more than 2 per molecule onaverage, characteristics of the ink composition for ink-jet recordingmay be reduced, for example, bleeding occurs.

In the present invention, the content of the phthalimidomethylatedquinacridone-based compound is preferably from 1 to 20% by mass, andmore preferably from 2 to 15% by mass, based on the total amount of thequinacridone-based pigment and the phthalimidomethylatedquinacridone-based compound. When the content of thephthalimidomethylated quinacridone-based compound is less than 1% bymass, the effect of using in combination may be exerted with difficulty.On the other hand, when the content is more than 20% by mass, storagestability of the pigment dispersion for ink-jet ink and ink compositionfor ink-jet recording may be drastically reduced.

Another one of the auxiliary dispersants used in the present inventionis a quinacridonesulfonic acid-based compound. As thequinacridonesulfonic acid-based compound, a conventionally known one canbe used. Examples thereof include quinacridonesulfonic acids obtained byreacting non-substituted quinacridone, dimethylquinacridone ordichloroquinacridone with concentrated sulfuric acid using a knownmethod; and metal salts thereof, such as sodium, aluminum and calciumsalts; and ammonium salts thereof, such as ammonium, octadecylammonium,didodecylammonium, dimethyloctadecylammonium,dimethyldioctadecylammonium and benzyldimethyloctadecylammonium salts.

Among these, alkali metal salt is preferable.

More specifically, the quinacridonesulfonic acid-based compound used inthe present invention is preferably an alkali metal salt of3,10-dichloroquinacridone sulfonic acid, and more preferably sodium3,10-dichloroquinacridonesulfonate.

In the present invention, the phthalimidomethylated quinacridone-basedcompound and the quinacridonesulfonic acid-based compound are notincluded in the quinacridone-based pigment.

(5) Humectant

As the humectant in the present invention, there can be used knownhumectants which have hitherto been used in the water-based ink forink-jet recording. Examples of the humectant include polyols such asglycerin, ethylene glycol, diethylene glycol, triethylene glycol,tetraethylene glycol, polyethylene glycol, propylene glycol, dipropyleneglycol, polypropylene glycol, 1,3-propanediol, 1,4-butanediol,1,5-pentanediol, 1,6-hexanediol, 1,2-hexanediol, 1,2,6-hexanetriol,trimethylolpropane and pentaerythritol; polyhydric alcohol alkyl etherssuch as diethylene glycol monobutyl ether, tetraethylene glycolmonomethyl ether and propylene glycol monoethyl ether; and polyhydricalcohol aryl ethers such as ethylene glycol monophenyl ether andethylene glycol monobenzyl ether.

These humectants usually serve as a poor solvent to the styrene-basedresin used in the present invention, and dispersibility of thestyrene-based resin during kneading is improved by adding the alkalimetal hydroxide. Therefore, a satisfactory solid and colored kneadedmixture containing the quinacridone-based pigment and the styrene-basedresin is formed by kneading in the presence of the humectant.

One or more kinds of these humectants can be used in combination. Whenusing a humectant having a boiling point of 170° C. or higher,preferably 200° C. or higher, the humectant is less likely to bevaporized during the kneading operation and kneading can be allowed toproceed while maintaining the solid content of the colored kneadedmixture. The humectant also serves as a humectant and a dryinginhibiting agent in the water-based pigment dispersion and thewater-based ink for ink-jet recording, and is preferably polyhydricalcohols having high boiling point, low volatility and high surfacetension, which are liquid at normal temperature, and more preferablyglycols such as diethylene glycol and triethylene glycol. By kneadingusing such a humectant, kneading can be conducted with goodreproducibility without causing vaporization of the humectant even inthe case of a kneading operation for a long period.

The respective steps of the following method of the present inventionwill now be described in detail.

(6) Method of Producing Water-based Pigment Dispersion

The kneading step and the dispersing step in the present invention aswell as the method of using auxiliary dispersants such asphthalimidomethylated quinacridone-based compound andquinacridonesulfonic acid-based compound will now be described indetail.

a. Kneading Step

In the kneading process of the present invention, a mixture containing apigment, a phthalimidomethylated quinacridone-based compound, astyrene-based resin having 60% by mass or more of a styrene-basedmonomer unit, a monomer unit containing an unsaturated aliphaticcarboxylic acid having a radical polymerizable double bond, an acidvalue of 50 to 300 and a weight-average molecular weight of 7500 to40000, an alkali metal hydroxide and a humectant is kneaded. In thekneading step, dispersibility is enhanced by neutralizing carboxylgroups in the styrene-based resin and also the resin is swollen by thehumectant thereby to soften the surface to form a mass made of a mixturetogether with the quinacridone-based pigment. Since the mixture is solidat normal temperature but is remarkably viscous at the kneadingtemperature of 50 to 90° C., a large shear force can be applied to themixture during kneading and the quinacridone-based pigment is crushedinto fine particles, while the surface of the fine particles are coatedwith the styrene-based resin. The phthalimidomethylatedquinacridone-based compound is effectively adsorbed onto the surface ofthe quinacridone-based compound crushed by kneading under a high shearforce.

In the kneading step, the kneading temperature (Mt) can be appropriatelyadjusted according to the temperature characteristics of thestyrene-based resin so as to enable kneading under a high shear force.Kneading is preferably carried out at the temperature which satisfiesthe following relationship:Tg−40≦Mt≦Tgwhere Mt is a kneading temperature and Tg is a glass transition point ofa styrene-based resin.

By kneading at the kneading temperature which satisfies the aboverelationship, kneading viscosity does not decrease by melting of theresin during kneading and the shear force does not decrease, resultingin sufficient kneading. Less liquid component is vaporized and the solidcontent of the colored kneaded mixture increases with difficulty afterthe completion of kneading. Furthermore, by kneading at the kneadingtemperature (Mt), which is at least 40° C. lower than the glasstransition point (Tg), the resin and the pigment are integrated at aninitial stage of the kneading and the subsequent kneading step proceedsremarkably efficiently. As used herein, the kneading temperature refersto a temperature of the inner surface of the vessel through which amixture is kneaded with the blade in the kneading step. The temperatureof the kneaded mixture itself reaches the temperature higher than thetemperature of the inner surface of the vessel by applying shear forceenergy. The kneaded mixture obtained after the completion of kneadingcontains a large content of a liquid component, and ground andencapsulated quinacridone-based pigment in the kneading step is easilydispersed in the aqueous medium in the subsequent dispersing step.Therefore, the resin, with which the pigment is coated, and theauxiliary dispersant adsorbed onto the pigment are less likely to beeliminated and they prevent aggregation of the quinacridone-basedpigment dispersed in the aqueous medium, thereby improving dispersionstability.

In the method of producing the water-based pigment dispersion of thepresent invention, it is preferred to use in combination with thequinacridonesulfonic acid-based compound as the auxiliary dispersant inview of improvement in dispersion stability, and thequinacridonesulfonic acid-based compound can be used with thephthalimidomethylated quinacridone-based compound in the kneading step.

In the kneading process of the method of producing the water-basedpigment dispersion of the present invention, since the styrene-basedresin is softened in the swollen state in the presence of an alkalimetal hydroxide and a humectant, the addition of a solvent fordissolving the resin and the process for distilling off the solventafter kneading are not required, and thus high production efficiency isattained.

The method of the present invention, capable of performing efficientkneading at the temperature lower than the glass transition temperatureof the resin, is suited for preparation of a water-based pigmentdispersion containing a resin having high Tg, which is considered to bekneaded with difficulty because it is not melted with ease, and is alsosuited for preparation of the water-based ink for ink-jet recording usedfor thermal jet type ink-jet recording. By using this method, a resinhaving high glass transition point suited for a thermal jet type inkcomposition for ink-jet recording can be kneaded at low temperature anddispersed in an aqueous medium, and thus a water-based pigmentdispersion having excellent thermostability can be prepared with ease.

In the kneading process of the present invention, in order to maintainthe solid content during kneading within a fixed value and to constantlyapply a stable shear force to the colored kneaded mixture, a close typekneader capable of suppressing vaporization of the humectant or akneader capable of serving as the close type kneader is preferably used,and a kneader equipped with a stirring tank, a lid of the stirring tank,and a monoxial or multiaxial stirring blade is more preferably used.Although the number of the stirring blade is not specifically limited,those having two or more stirring blades are preferably used so as toobtain high kneading action.

By using a kneader with such a configuration, after preparing a coloredkneaded mixture for water-based pigment dispersion through the kneadingstep, a water-based pigment dispersion can be prepared by directlydiluting the kneaded mixture in the same stirring tank without beingtaken out, and by subjecting to an initial dispersion step whilestirring, then to a final dispersion step.

Examples of the kneader include a Henschel mixer, pressurizing kneader,Banbury mixer and planetary mixer, and a planetary mixer is particularlypreferable. The planetary mixer refers to a planetary type kneadingapparatus and is a general term for a kneading apparatus having astirring blade for planetary movement (hereinafter referred to as aplanetary mixer).

In the method of the present invention, since a colored kneaded mixturehaving high solid content, which contains a pigment and a resin, iskneaded, the viscosity varies within a wide range according to thekneaded state of the kneaded mixture. Particularly, the planetary mixercan cope with the viscosity ranging from low viscosity to highviscosity, and the operations in the stage expanding from the initiationof the kneading process to the subsequent dilution which is thetransferring step to the dispersing process carried out in the sameapparatus. Furthermore, the humectant is added with ease and vacuumdistillation can be carried out, and also viscosity and shear force canbe adjusted with ease during kneading.

As described above, it is possible to gradually orient an anionichydrophilic group in the resin, wherein the surface of the pigment iscoated in the form of a capsule, to the direction of a peripheralaqueous medium while maintaining the encapsulated state by continuouslydiluting kneaded mixture manufactured at the kneading step, and thusrealizing the coated state which is excellent in wettability to theaqueous medium and is stable.

In the method of the present invention, in order to efficiently performkneading with the quinacridone-based pigment under high viscosity whilemaintaining the swollen state of the styrene-based resin, the solidcontent of the kneaded mixture containing the styrene-based resin, thequinacridone-based pigment, the phthalimidomethylated quinacridone-basedcompound and the quinacridonesulfonic acid-based compound duringkneading is preferably within a range from 50 to 80% by mass, and morepreferably from 60 to 80% by mass. When the solid content is less than50% by mass, since the viscosity of the mixture decreases, sufficientkneading may not be conducted and the pigment may not be crushedsufficiently. By maintaining the solid content within a range from 50 to80% by mass, reasonably high viscosity of the colored kneaded mixture ismaintained during kneading and a shear force applied to the coloredkneaded mixture by the kneader is increased, and thus grinding of thepigment in the colored kneaded mixture and coating of the pigment withthe resin are allowed to process simultaneously. When the solid contentis more than 80% by mass, even if the resin is sufficiently softened byheating, kneading may be carried out with difficulty and the coloredkneaded mixture may not be easily dispersed in the aqueous medium in thedispersing step.

In the kneading step, kneading may be conducted after appropriatelyadding water, in addition to the humectant.

In the case of preparing a colored kneaded mixture for ink-jet ink inthe kneading process of the present invention, the amount of thestyrene-based resin is preferably from 10 to 50 parts by mass, and morepreferably from 10 to 40 parts by mass, based on 100 parts by mass ofthe total amount of the quinacridone-based pigment, thephthalimidomethylated quinacridone-based compound and thequinacridonesulfonic acid-based compound in the colored kneaded mixture.When the amount of the styrene-based resin is less than 10 parts bymass, dispersion stability of the water-based pigment dispersion forink-jet ink may be reduced and, when the ink composition for ink-jetrecording is prepared, abrasion resistance of printed matter may bereduced. On the other hand, when the amount is more than 50 parts bymass, viscosity of the ink composition for ink-jet recording mayincrease excessively.

The alkali metal hydroxide used in the kneading step is added as anaqueous solution or an organic solvent solution of the alkali metalhydroxide. In this case, the content of the aqueous solution or organicsolvent solution of the alkali metal hydroxide is preferably from 20% to50% by mass. As the organic solvent which dissolves the alkali metalhydroxide, alcohol solvents such as methanol, ethanol and isopropanolare preferably used. Particularly, an aqueous solution of the alkalimetal hydroxide is preferably used in the method of the presentinvention.

The amount of the alkali metal hydroxide is 0.8 to 1.2 times the amountrequired to neutralize all carboxyl groups of the styrene-based resin.

The amount of the humectant in the kneading step is preferably within arange from 40 to 80 parts by mass based on 100 parts by mass of thetotal amount of the quinacridone-based pigment, thephthalimidomethylated quinacridone-based compound and thequinacridonesulfonic acid-based compound. When the amount of thehumectant is more than 80 parts by mass, the solid content decreases andthus it may become impossible to apply sufficient shear force. On theother hand, when the amount is less than 40% by mass, it becomesdifficult to fuse solids to form a mixture suited for kneading, and thusit may become impossible to apply sufficient shear force. Consequently,it may become difficult to sufficiently grind the quinacridone-basedpigment and adsorb the styrene-based resin onto the surface and thus auniform colored kneaded mixture for ink-jet ink may not be obtained.

The content of the humectant varies depending on the resin to be used,but is preferably from 10 to 50% by mass, and more preferably from 20 to40% by mass, based on the mixture. The amount of the humectant variesdepending on the humectant to be used, but is ½ to 5 times, preferably 2to 4.5 times, the amount of the resin. When the amount of the humectantis less than ½ times the amount of the resin, the resin cannot besoftened by converting into the swollen state suited for kneading andthus dispersion stability of the pigment may be reduced. On the otherhand, when the amount is more than 5 times, since the viscosity of themixture during kneading may be reduced and sufficient kneading cannot beconducted, dispersibility of the pigment may be reduced and thusdeterioration of image quality such as ejection failure may occur whenthe water-based ink for ink-jet recording is prepared.

b. Dispersing Step

The colored kneaded mixture for water-based pigment dispersion obtainedafter the completion of the kneading step is a kneaded mixture which issolid at normal temperature. In the dispersing step, this coloredkneaded mixture for ink-jet ink is diluted with an aqueous medium andthen subjected to a dispersion treatment to obtain a water-based pigmentdispersion for ink-jet ink. By subjecting to the dispersion treatment,coarse dispersed particles in the water-based pigment dispersion forink-jet ink are further ground and the particle size of dispersedparticles are diminished, and thus ink-jet characteristics such asejection stability and print density of the ink composition for ink-jetrecording are improved. In the method of the present invention, sincethe quinacridone-based pigment in the colored kneaded mixture forink-jet ink in the dispersing step has already been crushed in thekneading step and coated with the resin as the dispersant and,furthermore, auxiliary dispersants such as phthalimidomethylatedquinacridone-based compound and quinacridonesulfonic acid-based compoundare adsorbed onto the surface of the pigment, dispersibility to water isimproved. Therefore, the quinacridone-based pigment is easily dispersedin the aqueous medium within a short time and production efficiency isimproved.

In the method of producing the water-based pigment dispersion of thepresent invention, the quinacridonesulfonic acid-based compound may beused in combination, as the auxiliary dispersant, so as to improvedispersion stability and the quinacridonesulfonic acid-based compoundmay be added in the dispersing step. When the quinacridonesulfonicacid-based compound is added in the dispersing step, it is uniformlydispersed in the aqueous medium, together with the colored kneadedmixture which has been subjected to the kneading step, and then spreadover the surface of the quinacridone-based pigment and adsorbed onto thesurface.

When the quinacridonesulfonic acid-based compound is used in thedispersing step, preferably, only the colored kneaded mixture isdispersed in the aqueous medium and then the quinacridonesulfonicacid-based compound is added and the subsequent dispersing step isperformed.

When using the phthalimidomethylated quinacridone-based compound incombination with the quinacridonesulfonic acid-based compound,preferably, a colored kneaded mixture is prepared using both auxiliariesfrom the kneading step and then the kneaded mixture is dispersed in theaqueous medium to obtain a water-based pigment dispersion for ink-jetink because dispersion stability is more improved.

In the present invention, each amount of the phthalimidomethylatedquinacridone-based compound and the quinacridonesulfonic acid-basedcompound is 2 parts by mass based on 100 parts by mass of thequinacridone-based pigment. When using the phthalimidomethylatedquinacridone-based compound in combination with the quinacridonesulfonicacid-based compound, the total amount is preferably from 4 to 20 partsby mass, and more preferably from 4 to 15 parts by mass. When the amountis within the above range, the resulting water-based pigment dispersionfor ink-jet ink and ink composition for ink-jet recording are excellentin storage stability.

The content of the quinacridone-based pigment in the water-based pigmentdispersion for ink-jet ink is preferably from 5 to 25% by mass, and morepreferably from 5 to 20% by mass. When the content of thequinacridone-based pigment is less than 5% by mass, the ink compositionfor ink-jet recording prepared from the water-based pigment dispersionfor ink-jet ink may not be sufficiently colored and sufficient imagedensity may not be obtained. On the other hand, when the content is morethan 25% by mass, dispersion stability of the pigment may be reduced inthe water-based pigment dispersion for ink-jet ink.

In the present invention, the aqueous medium contains water or water anda humectant as a main component. The humectant used herein may be thesame as that used during kneading in the first step.

The disperser used in the dispersing step may be a known one, andexamples of the disperser using media include paint shaker, ball mill,Nano mill, attritor, basket mill, sand mill, sand grinder, Dinomill,Dispermat, SC mill, spike mill and agitator mill. Examples of thedisperser using no media include ultrasonic homogenizer, high-pressurehomogenizer, nanomizer, altimizer, dissolver, disper and high-speedimpeller disperser, and these dispersers may be used alone or incombination. Among these dispersers, dispersers using media arepreferable because of high dispersion capability. After dispersion, theconcentration may be adjusted with the aqueous medium, if necessary.

According to the kind of the disperser to be used, before subjecting todispersion (final dispersion) using the disperser, the viscosity ispreviously adjusted to viscosity suited for treatment using thedisperser by optionally adding an aqueous medium to the colored kneadedmixture, which has been subjected to the kneading step, followed bymixing and further dilution (hereinafter, a kneaded mixture of whichviscosity was adjusted may be referred to as a kneaded mixture havingadjusted viscosity). When using a kneading apparatus equipped with astirring tank and a stirring blade in the kneading step, the viscositycan be adjusted in the stirring tank before taking out the coloredkneaded mixture.

When using a sand mill, the kneaded mixture is preferably transferred tothe sand mill and then dispersed after adjusting the viscosity toseveral tens to several hundreds of mPa·sec by controlling the solidcontent to 10 to 40% by mass.

The aqueous medium used in the case of diluting the colored kneadedmixture may contain a humectant because it is necessary to preventdrying of the water-based pigment dispersion for ink-jet ink and toadjust the viscosity when a dispersion treatment is carried out. Thetotal content of the humectant used in the case of diluting the coloredkneaded mixture and that contained in the colored kneaded mixture forink-jet ink is preferably 3 to 50% by mass, and more preferably from 5to 40% by mass, based on the water-based pigment dispersion for ink-jetink. When the content is less than 3% by mass, the effect of inhibitingdrying may be lowered. On the other hand, when the content is more than50% by mass, dispersion stability of the dispersion may be lowered. Thehumectant used to prepare the colored kneaded mixture for ink-jet inkand the humectant contained in the aqueous medium which is used todilute the colored kneaded mixture may be the same or different.

(7) Preparation of Ink Composition for Ink-jet Recording

The ink composition for ink-jet recording of the present invention canbe prepared by further diluting the water-based pigment dispersion thusobtained with an aqueous medium. The content of the quinacridone-basedpigment is preferably from 2 to 10% by mass based on the ink compositionfor ink-jet recording prepared from the water-based pigment dispersionfor ink-jet ink of the present invention because it is necessary toobtain sufficient image density and to secure dispersion stability ofdispersed particles in the ink.

When the aqueous medium used to dilute the water-based pigmentdispersion preferably contains the humectant, the humectant contributesto prevention of drying, adjustment of the viscosity and adjustment ofthe content in the ink composition for ink-jet recording. The aqueousmedium includes, for example, the same aqueous medium as that used todisperse the colored kneaded mixture for the water-based pigmentdispersion. The content of the humectant used for the purpose inhibitingdrying is preferably from 3 to 50% by mass based on the ink.

In the case of preparing an ink composition for ink-jet recording,penetrants can be added for the purpose of improving permeability to themedium to be recorded and adjusting dot size on a recording medium.

Examples of the penetrant include lower alcohols such as ethanol andisopropyl alcohol; ethylene oxide adducts of alkyl alcohol, such asethylene glycol hexyl ether and diethylene glycol butyl ether; andpropylene oxide adducts of alkyl alcohol, such as propylene glycolpropyl ether. The content of the penetrant is preferably from 0.01 to10% by mass based on the ink.

In the case of preparing an ink composition for ink-jet recording,surfactants can be added so as to adjust ink properties such as surfacetension. Examples of the surfactant include, but are not limited to,various anionic surfactants, nonionic surfactants, cationic surfactantsand amphoteric surfactants. Among these surfactants, anionic surfactantsand nonionic surfactants are preferable.

Examples of the anionic surfactant include alkylbenzenesulfonic acidsalt, alkylphenylsulfonic acid salt, alkylnaphthalenesulfonic acid salt,higher fatty acid salt, sulfate ester salt of higher fatty acid ester,sulfonic acid salt of higher fatty acid ester, sulfate ester salt andsulfonic acid salt of higher alcohol ether, higher alkyl sulfosuccinicacid salt, polyoxyethylene alkyl ether carboxylic acid salt,polyoxyethylene alkyl ether sulfuric acid salt, alkyl phosphoric acidsalt and polyoxyethylene alkyl ether phosphoric acid salt. Specificexamples thereof include dodecylbenzenesulfonic acid salt,isopropylnaphthalenesulfonic acid salt,monobutylphenylphenolmonosulfonic acid salt, monobutylbiphenylsulfonicacid salt and dibutylphenylphenoldisulfonic acid salt.

Examples of the nonionic surfactant include polyoxyethylene alkyl ether,polyoxyethylene alkyl phenyl ether, polyoxyethylene fatty acid ester,sorbitan fatty acid ester, polyoxyethylene sorbitan fatty acid ester,polyoxyethylene sorbitol fatty acid ester, glycerin fatty acid ester,polyoxyethylene glycerin fatty acid ester, polyglycerin fatty acidester, sucrose fatty acid ester, polyoxyethylene alkylamine,polyoxyethylene fatty acid amide, fatty acid alkyloamide, alkylalkanolamide, acetylene glycol, oxyethylene adduct of acetylene glycoland polyethylene glycol-polypropylene glycol block copolymer. Amongthese surfactants, polyoxyethylene nonyl phenyl ether, polyoxyethyleneoctyl phenyl ether, polyoxyethylene dodecyl phenyl ether,polyoxyethylene alkyl ether, polyoxyethylene fatty acid ester, sorbitanfatty acid ester, polyoxyethylene sorbitan fatty acid ester, fatty acidalkylolamide, acetylene glycol, oxyethylene adduct of acetylene glycoland polyethylene glycol-polypropylene glycol block copolymer arepreferable.

As the other surfactant, for example, there can be used silicone-basedsurfactants such as polysiloxaneoxyethylene adduct; fluorine surfactantssuch as perfluoroalkylcarboxylic acid salt, perfluoroalkylsulfonic acidsalt and oxyethylene perfluoroalkyl ether; and biosurfactants such asspiculisporic acid, rhamnolipid and lysolecithin. These surfactants canbe used alone or in combination. Taking account of dissolution stabilityof the surfactant, HLB is preferably within a range from 7 to 20.

When the surfactant is added, the content is preferably within a rangefrom 0.001 to 1% by mass, more preferably from 0.001 to 0.5% by mass,and most preferably from 0.01 to 0.2% by mass, based on the entire massof the ink composition for ink-jet recording. When the content of thesurfactant is less than 0.001% by mass, the effect of adding thesurfactant may not be exerted. On the other hand, when the content ismore than 1% by mass, there may cause problems such as bleeding.

In the case of preparing an ink composition for ink-jet recording usingthe water-based pigment dispersion for ink-jet ink of the presentinvention, antiseptics, viscosity adjustors, pH adjustors, chelatingagents, plasticizers, antioxidants and ultraviolet absorbers can beadded, if necessary.

In the case of preparing an ink composition for ink-jet recording, sincecoarse particles can cause clogging of a nozzle and deterioration ofother image quality, the coarse particles may be removed by centrifugalseparation or filtration treatment after preparing the ink.

The ink composition for ink-jet recording can be preferably used as anink for ink-jet recording. Examples of the ink-jet system to which theink composition is applied include, but are not limited to, knownsystems, for example, continuous systems such as charge control typesystem and spray type system; and drop on-demand type systems such aspiezo type system, thermal type system and electrostatic attraction typesystem. It is preferred to apply to a thermal ink-jet recording typeprinter.

EXAMPLES

The present invention will now be described in detail by way ofexamples. In the examples, parts and percentages are by mass unlessotherwise specified. Resins used in the examples and comparativeexamples are as follows.

-   Resin A: resin including monomers in a ratio of styrene/acrylic    acid/methacrylic acid=77/10/13 (mass ratio) having a weight-average    molecular weight of 7700, an acid value of 151 mgKOH/g and a glass    transition point of 107° C.-   Resin B: resin including monomers in a ratio of styrene/acrylic    acid/methacrylic acid=77/10/13 (mass ratio) having a weight-average    molecular weight of 11000, an acid value of 152 mgKOH/g and a glass    transition point of 107° C.-   Resin C: resin including monomers in a ratio of styrene/acrylic    acid/methacrylic acid=77/10/13 (mass ratio) having a weight-average    molecular weight of 20000, an acid value of 151 mgKOH/g and a glass    transition point of 107° C.-   Resin D: resin including monomers in a ratio of styrene/methacrylic    acid=77/23 (mass ratio) having a weight-average molecular weight of    10700, an acid value of 146 mgKOH/g and a glass transition point of    116° C.-   Resin E: resin including monomers in a ratio of styrene/acrylic    acid/methacrylic acid=77/10/13 (mass ratio) having a weight-average    molecular weight of 5000, an acid value of 150 mgKOH/g and a glass    transition point of 107° C.-   Resin F: resin including monomers in a ratio of styrene/acrylic    acid/methacrylic acid=77/10/13 (mass ratio) having a weight-average    molecular weight 45000, an acid value of 153 mgKOH/g and a glass    transition point of 107° C.-   Resin G: resin including monomers in a ratio of styrene/methyl    methacrylate/acrylic acid/methacrylic acid=50/27/10/13 (mass ratio)    having a weight-average molecular weight of 12000, an acid value of    149 mgKOH/g and a glass transition point of 109° C.

To confirm the effect exerted when phthalimidomethylated3,10-dichloroquinacridone is used as a phthalimidomethylatedquinacridone-based compound, water-based pigment dispersions shown inthe following examples and comparative examples were prepared.

Example 1

A mixture with the following composition was charged in a planetarymixer (manufactured by Inoue Seisakusho under the trade name of“PLM-V-50V”) having a volume of 50 L and, after heating a jacket, themixture was kneaded at a low speed (rotation speed: 21 rpm, revolutionspeed: 14 rpm) until the temperature of contents reached 60° C. Afterchanging to high speed (rotation speed: 35 rpm, revolution speed: 24rpm), kneading was continued.

Resin A 750 g Quinacridon-based pigment (manufactured by DAINIPPON 4630g  INK AND CHEMICALS, INCORPORATED under the trade name of “FASTOGENSUPER MAGENTA RTS”) Phthalimidomethylated 3,10-dichloroquinacridone(average 380 g number of phthalimidemethyl group per molecule: 1.4)Diethylene glycol 2700 g  Aqueous 34% by mass potassium hydroxidesolution 333 g Ion-exchanged water 200 g

On changing to high speed, a current value of a planetary mixer was 5 A.After kneading was continued, a maximum current value of the planetarymixer was 15 A. After kneading was continued for one hour since themaximum current value was attained, the current value of the planetarymixer was 10 A. To the resulting kneaded mixture in a stirring tank, 200g of ion-exchanged water heated to 60° C. was added and kneading wascontinued. Uniform mixing was confirmed and, after an additional onehour, 200 g of ion-exchanged water heated to 60° C. was added, followedby uniform mixing and further continuous kneading.

Kneading was continued for 4 hours since the observation of the maximumcurrent value to obtain a colored resin composition. To the resultingcolored resin composition, 6400 g of ion-exchanged water heated to 60°C. was added in several portions over 2 hours while stirring using aplanetary mixer.

The solid content of the colored resin composition after diluting withwater was 39.0% by mass. To 12 kg of the colored resin composition takenafter diluting with water, 5.83 kg of diethylene glycol and 5.57 kg ofion-exchanged water were added by several portions while stirring usinga dispersion stirrer to obtain a water-based pigment dispersionprecursor A-1.

18 kg of the water-based pigment dispersion precursor A-1 was dispersedby using a bead mill (manufactured by ASADA IRON WORKS CO., LTD. underthe trade name of “Nano Mill NM-G2L”) under the following conditions toobtain a pigment dispersion A-2.

Dispersion Conditions

-   Disperser: manufactured by ASADA IRON WORKS CO., LTD. under the    trade name of “Nano millNM-G2L”-   Beads: 0.3 mm in diameter zirconia beads-   Amount of beads filled: 85%-   Cooling water temperature: 10° C.-   Rotation speed: 2660 rpm-   (Disk peripheral speed: 12.5 m/sec)-   Delivery amount: 200 g/10 sec

A dispersion treatment was conducted while circulating for one hourunder the above conditions. The content of the quinacridone-basedpigment of the pigment dispersion A-2 was 15.8% by mass.

Example 2 to Example 4, Comparative Example 1 and Comparative Example 3to Comparative Example 5

In substantially the same manner as in Example 1, except for the kind ofthe resin and use of phthalimidomethylated 3,10-dichloroquinacridone inaccordance with the formulation shown in Table 1, water-based pigmentdispersions B-2, C-2, D-2, E-2, G-2, H-2 and I-2 of Example 2 to Example4, Comparative Example 1 and Comparative Example 3 to ComparativeExample 5 were prepared. Details of the amount are as shown in Table 1.

TABLE 1 Example 1 Example 2 Example 3 Example 4 Resin A B C D Amount ofresin 750 750 1000 750 FASTOGEN SUPERMAGENTA RTS 4630 4630 4750 4630Phthalimidomethylated 3,10-dichloroquinacridone 380 380 250 380Quinacridonsulfonic acid-based compound Diethylene glycol 2700 2900 32003100 Aqueous 34% by mass potassium hydroxide solution 333 335 489 322Ion-exchanged water 200 Additional ion-exchanged water for kneading 400800 300 300 Additional ion-exchanged water for dilution 6400 6200 67006700 Solid content of colored kneaded mixture after dilution   39% 39.9%38.5% 38.1% Colored kneaded mixture after dilution 12000 12000 1200012000 Diethylene glycol 5830 5810 1350 1490 Ion-exchanged water 55706150 9750 9340 Water-based pigment dispersion before dispersiontreatment A-1 B-1 C-1 D-1 Supply flow rate of dispersion to disperser200 g/10 sec 185 g/10 sec 190 g/10 sec 195 g/10 sec Content of pigmentof water-based pigment dispersion 15.8% 15.8% 15.4% 15.8% Water-basedpigment dispersion A-2 B-2 C-2 D-2 Comparative Comparative ComparativeComparative Example 1 Example 3 Example 4 Example 5 Resin B E F G Amountof resin 1000 750 750 750 FASTOGEN SUPERMAGENTA RTS 5000 4630 4630 4630Phthalimidomethylated 3,10-dichloroquinacridone 380 380 380Quinacridonsulfonic acid-based compound Diethylene glycol 3200 2500 33003000 Aqueous 34% by mass potassium hydroxide solution 447 331 338 329Ion-exchanged water 200 Additional ion-exchanged water for kneading 200200 400 500 Additional ion-exchanged water for dilution 6000 6800 64006500 Solid content of colored kneaded mixture after dilution 40.4%   39%38.4% 39.2% Colored kneaded mixture after dilution 12000 12000 1200012000 Diethylene glycol 1420 2000 1340 1610 Ion-exchanged water 112509400 9700 9910 Water-based pigment dispersion before dispersiontreatment E-1 G-1 H-1 I-1 Supply flow rate of dispersion to disperser200 g/10 sec 200 g/10 sec 195 g/10 sec 200 g/10 sec Content of pigmentof water-based pigment dispersion   16% 15.8% 15.8% 15.8% Water-basedpigment dispersion E-2 G-2 H-2 I-2 *unit is g unless otherwise soindicated

When high-speed kneading is conducted by using a planetary mixer duringkneading in accordance with the above formulation, the temperature ofthe kneaded mixture was from 90 to 95° C.

In Comparative Example 2, a water-based pigment dispersion was preparedwithout using the kneading step.

Comparative Example 2

Preparation of Aqueous Resin Solution

In accordance with the following formulation, a methyl ethyl ketonesolution of a resin B was prepared.

Methyl ethyl ketone (abbreviated as MEK) 50 g Resin B 50 g

To this was added a mixed solution of 247.6 g of ion-exchanged water and22.35 g of an aqueous 34% by mass potassium hydroxide solution, followedby sufficient stirring to obtain a resin B solution.

MEK with water were distilled off from the resin B solution underreduced pressure condition of a water bath temperature of 35° C. and apressure of 20 hPa. After the completion of distillation of MEK,ion-exchanged water was added to obtain an aqueous resin B solutionhaving a solid content of 20% by mass.

Dispersion of Pigment

In a 250 ml polyethylene bottle, 400 g of 1.2 mm in diameter zirconiabeads were placed and the following components were treated by a paintconditioner manufactured by Toyo Seiki Seisaku-Sho, Ltd. for 4 hours.

Aqueous resin B solution 8.64 g Quinacridon-based pigment (manufacturedby DAINIPPON 9.25 g INK AND CHEMICALS, INCORPORATED under the trade nameof “FASTOGEN SUPER MAGENTA RTS”) Phthalimidomethylated3,10-dichloroquinacridone (average 0.75 g number of phthalimidemethylgroup per molecule: 1.4) Diethylene glycol 20.0 g Ion-exchanged water20.0 g

After the completion of the treatment, 10.33 g of ion-exchanged waterwas added. After further treating for 30 minutes, beads were removed byfiltration to obtain a pigment dispersion F-1.

The content of the quinacridone-based pigment of the pigment dispersionF-1 was 13.4% by mass.

To confirm the effect exerted when phthalimidomethylated3,10-dichloroquinacridone is used as a phthalimidomethylatedquinacridone-based compound and a quinacridonesulfonic acid-basedcompound is used in combination, water-based pigment dispersions shownin the following examples were prepared.

Example 5

Kneading Conditions

A mixture with the following composition was charged in a planetarymixer and then kneaded under the same conditions as in Example 1.

Resin B  750 g Quinacridon-based pigment (manufactured by DAINIPPON INK4250 g AND CHEMICALS, INCORPORATED under the trade name of “FASTOGENSUPER MAGENTA RTS”) Compound of the formula (III) (average number of 250 g phthalimidemethyl group: 1.4) Hydrous cake having a solid contentof 28.7% of 3,10- 1742 g dichloroquinacridone sulfonic acid aluminumsalt (average number of sulfone group: 1.3) Diethylene glycol 2000 gAqueous 34% by mass potassium hydroxide solution  335 g

On changing to high speed, a current value of a planetary mixer was 5 A.After kneading was continued, a maximum current value of the planetarymixer was 15 A. After kneading was continued for 3 hours since themaximum current value was attained, the current value of the planetarymixer was 12 A. To the resulting kneaded mixture in a stirring tank, 200g of ion-exchanged water heated to 60° C. was added and kneading wascontinued. Uniform mixing was confirmed and, after additional 30minutes, 200 g of ion-exchanged water heated to 60° C. was added,followed by uniform mixing and further continuous kneading.

Kneading was continued for 4 hours since the observation of the maximumcurrent value to obtain a colored resin composition. To the resultingcolored resin composition, 5600 g of ion-exchanged water heated to 60°C. was added by several portions over 2 hours while stirring using aplanetary mixer.

The solid content of the colored resin composition after diluting withwater was 40.6% by mass. To 12 kg of the colored resin composition takenafter diluting with water, 2.49 kg of diethylene glycol and 9.98 kg ofion-exchanged water were added by several portions while stirring usinga dispersion stirrer to obtain a water-based pigment dispersionprecursor. 18 kg of the water-based pigment dispersion precursor J-1 wasdispersed by using a beads mill under the same conditions as in Example1 to obtain a pigment dispersion J-2. The content of thequinacridone-based pigment of the pigment dispersion J-2 was 14.5% bymass.

Example 6 and Example 7

Using 3,10-dichloroquinacridone sulfonic acid sodium salt in the form ofa hydrous cake having a solid content of 28.6% in place of3,10-dichloroquinacridone sulfonic acid aluminum salt, a water-basedpigment dispersion K-2 was prepared under almost the same conditions asin Example 5. Using a resin D in place of the resin B, a water-basedpigment dispersion L-2 was prepared under almost the same conditions asin Example 5. Details of the amounts are as shown in Table 2.

Example 8

Kneading Conditions

A mixture with the following composition was charged in a planetarymixer and then kneaded under the same conditions as in Example 1.

Resin B 750 g Quinacridon-based pigment (manufactured by DAINIPPON INK4750 g  AND CHEMICALS, INCORPORATED under the trade name of “FASTOGENSUPER MAGENTA RTS”) Compound of the formula (III) (average number of 250g phthalimidemethyl group: 1.4) Diethylene glycol 3000 g  Aqueous 34% bymass potassium hydroxide solution 335 g

On changing to high speed, a current value of a planetary mixer was 5 A.After kneading was continued, a maximum current value of the planetarymixer was 12 A. After kneading was continued for 1.5 hours since themaximum current value was attained, the current value of the planetarymixer was 10 A. To the resulting kneaded mixture in a stirring tank, 200g of ion-exchanged water heated to 60° C. was gradually added andkneading was continued.

Kneading was continued for 4 hours since the observation of the maximumcurrent value to obtain a colored resin composition. To the resultingcolored resin composition, 6800 g of ion-exchanged water heated to 60°C. was added by several portions over 2 hours while stirring using aplanetary mixer. The solid content of the colored resin compositionafter diluting with water was 39.4% by mass. To 12 kg of the coloredresin composition taken after diluting with water, 1.61 kg of diethyleneglycol and 10.02 kg of ion-exchanged water were added by severalportions while stirring using a dispersion stirrer to obtain awater-based pigment dispersion precursor M-1.

18 kg of the water-based pigment dispersion precursor M-1 was subjectedto a dispersion treatment by using a beads mill under the sameconditions as in Example 1 to obtain a pigment dispersion M-2. Thecontent of the quinacridone-based pigment of the pigment dispersion M-2was 16.2% by mass.

The water-based pigment dispersion precursor M-2 was subjected to adispersion treatment under the following conditions to obtain awater-based pigment dispersion M-3.

Dispersion Conditions (2)

In a 250 ml polyethylene bottle, 200 g of 1.2 mm in diameter zirconiabeads were placed and the following components were treated by a paintconditioner manufactured by Toyo Seiki Seisaku-Sho, Ltd. for 30 minutesto obtain a water-based pigment dispersion M-3. The content of thequinacridone-based pigment in the resulting water-based pigmentdispersion M-3 was 12.8%.

Water-based pigment dispersion precursor M-2 62.96 g Hydrous slurryhaving a solid content of 10.0% of 3,10- 12.63 g dichloroquinacridonesulfonic acid sodium salt (average number of sulfone groups: 1.3)Ion-exchanged water  4.41 g (Example 9)

Using a resin D in place of the resin B, a water-based pigmentdispersion N-3 was prepared under almost the same conditions as inExample 9. Details of the amount are as shown in Table 2.

TABLE 2 Example 5 Example 6 Example 7 Example 8 Example 9 Kneading ResinB B D B D Amount of resin 750 750 750 750 750 FASTOGEN SUPERMAGENTA RTS4250 4250 4250 4750 4630 Phthalimidomethylated 3,10- 250 250 250 250 380dichloroquinacridone Hydrous cake of 1742 1746 1746 quinacridonesulfonicacid- based compound Content of hydrous cake 28.7% 28.6% 28.6%Diethylene glycol 2000 2000 2000 3000 3100 Aqueous 34% by mass potassium335 335 322 335 332 hydroxide solution Additional ion-exchanged water400 200 200 300 for kneading Additional diethylene glycol 200 forkneading Dilution Additional ion-exchanged water 5600 6000 6000 68006700 for dilution Solid content of colored 40.6%   42% 41.2% 39.4% 38.1%kneaded mixture after dilution Dispersion I Colored kneaded mixtureafter 12000 12000 12000 12000 12000 dilution Diethylene glycol 2490 24102360 1610 1490 Ion-exchanged water 9880 10820 10330 10020 9340Water-based pigment dispersion J-1 K-1 L-1 M-1 N-1 before dispersiontreatment Delivery amount of disperser 200 g/10 sec 190 g/10 sec 200g/10 sec 200 g/10 sec 200 g/10 sec Content of pigment of water- 14.5%14.5% 14.5% 16.2% 15.8% based pigment dispersion Water-based pigmentdispersion J-2 K-2 L-2 M-2 N-2 Dispersion Water-based pigment dispersion62.96 66.55 II 10% by mass 12.63 6.38 quinacridonesulfonic acid- basedcompound Ion-exchanged water 4.41 7.06 Content of pigment of 12.8% 13.1%dispersion after dispersion II Water-based pigment dispersion M-3 N-3after dispersion II *unit is g unless otherwise so indicated

When high-speed kneading is conducted by using a planetary mixer duringkneading in accordance with the above formulation, the temperature ofthe kneaded mixture was from 90 to 95° C.

(Evaluation of Water-based Pigment Dispersion)

With respect to the water-based pigment dispersions thus obtained in theExamples and Comparative Examples, the particle size was measured at acell temperature of 25° C. by using a Microtrac UPA particle sizeanalyzer manufactured by Leeds & Northrup Co. In that case, samples formeasurement of the particle size were prepared by diluting each of theresulting water-based pigment dispersions with ion-exchanged water so asto adjust the content of the quinacridone-based pigment to 12.5%, andfurther diluting with ion-exchanged water by 500 times. The results areshown in Table 3 and Table 4.

(Evaluation of Dispersion Stability)

In the same manner as in the case of the dispersion, the water-basedpigment dispersions of the Examples and Comparative Examples wereadjusted with ion-exchanged water after diluting with ion-exchangedwater so as to adjust the content of the quinacridone-based pigment to12.5% by mass. After controlling the content of the pigment, thedispersion was sealed in a glassware such as screw-thread glass vial andsubjected to a heating test in a thermostatic chamber at 60° C. for oneweek. The dispersion stability was evaluated by observing a change inparticle size before and after the heating test. The results are shownin Table 3 and Table 4.

TABLE 3 Weight- Volume- Volume-average Composition average averageparticle size of resin molecular particle after heating ChangeDispersion Resin No. St/MMA/AA/MAA weight size (nm) test (nm) ratioExample 1 A-2 A 77/0/10/13 11000 108 112 4% Example 2 B-2 B 77/0/10/1311000 106 103 −3%   Example 3 C-2 C 77/0/10/13 20000 116 116 0% Example4 D-2 D 77/0/0/23 10700 125 134 7% Comparative E-2 B 77/0/10/13 11000127 142 12%  Example 1 Comparative F-1 B 77/0/10/13 11000 137 145 6%Example 2 Comparative G-2 E 77/0/10/13 5000 107 116 8% Example 3Comparative H-2 F 77/0/10/13 45000 118 115 −3%   Example 4 ComparativeI-2 G 50/27/10/13 12000 115 117 2% Example 5

In Table 3, a change in dispersibility of the water-based pigmentdispersions due to the addition of phthalimidomethylated3,10-dichloroquinacridone and change of the styrene-based resin isshown. The water-based pigment dispersion of Comparative Example 1wherein phthalimidomethylated 3,10-dichloroquinacridone is not added isinferior in dispersion stability during heating. When using astyrene-based resin obtained by copolymerizing three kinds of monomerssuch as styrene, acrylic acid, methacrylic acid, the resultingwater-based pigment dispersion is excellent in dispersion stability.Furthermore, when using a styrene-based resin having a higherweight-average molecular weight, the resulting water-based pigmentdispersion is excellent in stability.

TABLE 4 Weight- Volume- Volume-average Composition average averageparticle size of resin molecular particle after heating ChangeDispersion Resin No. St/MMA/AA/MAA weight size (nm) test (nm) ratioExample 5 J-2 B 77/0/10/13 11000 120 126 5% Example 6 K-2 B 77/0/10/1311000 117 119 2% Example 7 L-2 D 77/0/0/23 10700 115 115 0% Example 8M-3 B 77/0/10/13 11000 105 108 3% Example 9 N-3 D 77/0/0/23 10700 127130 2%

In Table 4, the effect of the use of phthalimidomethylated3,10-dichloroquinacridone in combination of the quinacridonesulfonicacid-based compound exerted on the dispersibility of the water-basedpigment dispersion is shown. As is apparent from the results shown inTable 4, the resulting water-based pigment dispersions show excellentdispersion stability.

To each of the water-based pigment dispersions obtained in therespective Examples and Comparative Examples, ion-exchanged water wasadded to obtain 50.0 g of a diluted pigment solution containing 10% bymass of the quinacridone-based pigment. Using the resulting dilutedwater-based pigment solution, an ink composition was prepared so as toadjust the content of the quinacridone-based pigment to 5% in accordancewith the following two kinds of formulations in which surfactants havingdifferent HLB values and different hydrophobicities are used. In thecase of comparing the ink formulation A with the ink formulation B,dispersion stability of the ink composition varies depending on aninfluence of the surfactant, and the ink composition prepared inaccordance with the ink formulation B is more stable.

(Ink formulation A) Diluted water-based pigment solution 50.0 g2-pyrrolidinone 8.0 g Triethylene glycol monobutyl ether 8.0 g Glycerin3.0 g Surfactant (manufactured by Air Products Co., Ltd. under the 0.5 gtrade name of “SURFYNOL 440”) Ion-exchanged water 30.5 g (Inkformulation B) Diluted water-based pigment solution 50.0 g2-pyrrolidinone 8.0 g Triethylene glycol monobutyl ether 8.0 g Glycerin3.0 g Surfactant (manufactured by Air Products Co., Ltd. under the 0.5 gtrade name of “SURFYNOL 465”) Ion-exchanged water 30.5 g

Reference Example

(Comparison of Influence of Ink Formulation A and Ink Formulation BExerted on Ink Composition)

A difference in dispersion stability between the ink formulation A andthe ink formulation B will now be compared.

After diluted water-based pigment solution was prepared from thewater-based pigment dispersion B-2 obtained in Example 2, two kinds ofink compositions were prepared in accordance with the ink formulation Aand ink formulation B. The resulting ink composition was sealed in aglassware such as screw-thread glass vial and subjected to a heatingtest in a thermostatic chamber at 70° C. for one week. The stability wasevaluated by observing a change in particle size and viscosity beforeand after the heating test. The results are shown in Table 5 and Table6. As is apparent from the results, the dispersion of the inkcomposition prepared in accordance with the ink formulation B is morestable.

The particle size was measured in the same manner as in the case ofevaluation of the water-based pigment dispersion, except that the inkcomposition was diluted with ion-exchanged water by 100 times. Theviscosity was measured by an E-type viscometer (manufactured by TokyoKeiki Co., Ltd., under the trade name of “VISCOMETER TV-20”) at 25° C.

Consequently, a conventional ink composition was evaluated by using theink formulation B, while an ink composition having excellent dispersionstability was evaluated by using the ink formulation A so as to clarifythe effect.

TABLE 5 Volume-average Volume-average particle size particle sizeViscosity before Viscosity after before heating after heating Changeheating test heating test Change test (nm) test (nm) ratio (mPa · s)(mPa · s) ratio Ink 98 111 13% 3.08 3.80 23% formulation A Ink 98 101 2% 3.26 3.32  2% formulation B(Stability Test of Ink Composition)

The resulting ink composition was sealed in a glassware such asscrew-thread glass vial and subjected to a heating test in athermostatic chamber at 70° C. for one week. The stability of the inkcomposition was evaluated by observing a change in particle size andviscosity before and after the heating test.

The particle size was measured in the same manner as in the case ofevaluation of the water-based pigment dispersion, except that the inkcomposition was diluted with ion-exchanged water by 100 times. Theviscosity was measured by an E-type viscometer (manufactured by TokyoKeiki Co., Ltd., under the trade name of “VISCOMETER TV-20”) at 25° C.

The results are shown in Table 6.

(Evaluation of Ink-jet Recordability)

The resulting ink composition before subjecting to the heating test wasinjected in a black pen of a thermal jet type ink-jet printermanufactured by Hewlett-Packard Co., Ltd. under the trade name of“DeskJet957C”, and then the printing test was carried out.

Specifically, solid printing and fine-line printing were carried outusing A4-size paper and the ejection state of the ink was observed.

The results are shown in Table 6. Visual observation was carried outaccording to the following criteria.

With respect to evaluation of ink-jet recordability:

-   A: uniform solid printing, ejection failure was not observed even at    the fine-line portion in all printed samples-   B: slightly non-uniform solid printing, ejection failure was not    observed even at the fine-line portion-   C: non-uniform solid printing, ejection failure was not observed at    the fine-line portion but printing position deviation was observed-   D: irregular printing due to ejection failure was observed at the    solid printing portion, and printing omission due to ejection    failure was observed at the fine-line portion-   E: ejection failure often occurs

In the same manner, the resulting ink composition before subjecting tothe heating test was injected in a black pen of a piezoelectric jet typeprinter manufactured by Epson Corp. under the trade name of “EM-900C”,and then the printing test was carried out.

Specifically, solid printing and fine-line printing were carried outusing A4-size paper and the ejection state of the ink was observed. Inthe case of both ink compositions of the Examples and ComparativeExamples, no problems were observed.

TABLE 6 Resin Pigment derivative Weight-average Sulfonic molecularacid-based Kneading Dispersing Dispersion Kind Composition weightFormula I compound step step Example 5 J-2 B 77/10/13 11000 ∘ ∘ ∘ onestage Example 6 K-2 B 77/10/13 11000 ∘ ∘ ∘ one stage Example 7 L-2 D77/23 10700 ∘ ∘ ∘ one stage Example 8 M-3 B 77/10/13 11000 ∘ ∘ ∘ twostages Example 9 N-3 D 77/23 10700 ∘ ∘ ∘ two stages Reference B-2 B77/10/13 11000 ∘ ∘ one stage Example Example 1 A-2 A 77/10/13 11000 ∘ ∘one stage Example 2 B-2 B 77/10/13 11000 ∘ ∘ one stage Example 3 C-2 C77/10/13 20000 ∘ ∘ one stage Example 4 D-2 D 77/23 10700 ∘ ∘ one stageComparative E-2 B 77/10/13 11000 ∘ one stage Example 1 Comparative F-2 B77/10/13 11000 ∘ one stage Example 2 Comparative G-2 E 77/10/13 5000 ∘ ∘one stage Example 3 Comparative H-2 F 77/10/13 45000 ∘ ∘ one stageExample 4 Comparative I-2 G 50/27/10/13 12000 ∘ ∘ one stage Example 5Particle size (nm) Viscosity (mPa · sec) Before After Before After Inkheating heating Change heating heating Change Ink-jet formulation testtest ratio test test ratio recordability Example 5 A 107 110 3 3.14 3.212 B Example 6 A 110 108 −2 3.03 3.03 0 A Example 7 A 120 124 3 3.01 2.785 B Example 8 A 110 106 −4 4.19 3.03 −28 A Example 9 A 125 135 8 3.63.85 7 B Reference A 98 111 13 3.08 3.8 23 Example Example 1 B 102 108 63.15 3.24 3 A Example 2 B 99 101 2 3.26 3.32 2 A Example 3 B 107 108 13.22 3.21 0 A Example 4 B 121 128 6 3.18 3.2 1 B Comparative B 125 24394 2.97 4.85 63 D Example 1 Comparative B 129 215 67 3.42 7.77 127 CExample 2 Comparative B 111 122 10 2.81 3.24 15 B Example 3 ComparativeB 118 118 0 4.12 4.02 −2 E Example 4 Comparative B 115 118 3 3.01 3.05 1C Example 5

As is apparent from the results shown in Table 6, when water-based inkcompositions are prepared from the water-based pigment dispersions ofExample 5 to Example 9 wherein phthalimidomethylated3,10-dichloroquinacridone and a quinacridonesulfonic acid-based compoundare used in combination, the resulting ink compositions are excellent indispersion stability during heating and ink-jet recordability.

The ink compositions of Example 1 to Example 4, wherein onlyphthalimidomethylated 3,10-dichloroquinacridone is used, are excellentin characteristics. However, the ink compositions of Example 5 toExample 9 are more excellent in dispersion stability during heatingtaking accounts of the ink formulation. The ink composition ofComparative Example 1, wherein no phthalimidomethylatedquinacridone-based compound is used, showed large increase in particlesize and viscosity before and after the heating and is inferior instability during heating. The ink composition of Comparative Example 2,wherein the phthalimidomethylated quinacridone-based compound is usedbut the kneading step is not included, also showed large increase inparticle size and viscosity before and after the heating and is inferiorin stability during heating. The ink composition of Comparative Example3 is excellent in ink-jet recordability, but is inferior in stabilityduring heating as compared with those of Example 1 to Example 4 becauseof the influence of the weight-average molecular weight of the resin.The ink composition of Comparative Example 4, wherein a resin having alarge weight-average molecular weight is used, is excellent indispersion stability, but is inferior in recordability due to a thermaljet type ink-jet printer. The ink composition of Comparative Example 5,wherein a four-component resin containing 50% by mass or more of astyrene-based monomer unit, is not inferior in dispersion stability, butis inferior in excellent recording characteristics as compared withthose of Example 1 to 4.

INDUSTRICAL APPLICABILITY

According to the present invention, it is made possible to efficientlyobtain a water-based ink for ink-jet recording, which contains pigmentparticles having a very small particle size and is excellent indispersion stability, by further diluting the resulting water-basedpigment dispersion with an aqueous medium and optionally adding variousadditives, and thus the present invention is very effective fromindustrial points of view.

1. A method of producing a water-based pigment dispersion for ink-jetink, comprising: a kneading process for kneading a mixture containing astyrene-based resin, a quinacridone-based pigment, aphthalimidomethylated quinacridone-based compound, an alkali metalhydroxide and a humectant to produce a solid and colored kneadedmixture; and a dispersing process for dispersing the solid and coloredkneaded mixture in an aqueous medium, wherein the styrene-based resinhas 60% by mass or more of a styrene-based monomer unit based on allmonomer components, a monomer unit containing an unsaturated aliphaticcarboxylic acid having a radical polymerizable double bond, an acidvalue of 50 to 300 and a weight-average molecular weight of 7500 to40000; and the solid content of the kneaded mixture containing thestyrene-based resin, the quinacridone-based pigment and thephthalimidomethylated quinacridone-based compound during kneading isfrom 50 to 80% by mass.
 2. The method of producing a water-based pigmentdispersion for ink-jet ink according to claim 1, wherein aquinacridonesulfonic acid-based compound is added in the kneading ordispersing process.
 3. The method of producing a water-based pigmentdispersion for ink-jet ink according to claim 1, wherein thestyrene-based resin has 60% by mass or more of a styrene-based monomerunit based on all monomer components, an acrylic acid monomer unit and amethacrylic acid monomer unit.
 4. The method of producing a water-basedpigment dispersion for ink-jet ink according to claim 1 or 2, wherein,in the kneading process, the content of the styrene-based resin is from10 to 50% by mass based on 100 parts by mass of the total amount of thequinacridone-based pigment, the phthalimidomethylated quinacridone-basedcompound and the quinacridonesulfonic acid-based compound; and thecontent of the humectant is from 40 to 80 parts by mass based on 100parts by mass of the total amount of the quinacridone-based pigment, thephthalimidomethylated quinacridone-based compound and thequinacridonesulfonic acid-based compound.
 5. The method of producing awater-based pigment dispersion for ink-jet ink according to claim 1 or2, wherein the styrene-based resin has a glass transition point of 90°C. or higher.
 6. The method of producing a water-based pigmentdispersion for ink-jet ink according to claim 1 or 2, wherein the amountof the alkali metal hydroxide is 0.8 to 1.2 times the amount required toneutralize all carboxyl groups of the styrene-based resin.
 7. The methodof producing a water-based pigment dispersion for ink-jet ink accordingto claim 1, wherein the phthalimidomethylated quinacridone-basedcompound is a compound represented by the formula (I):

wherein R and R′ each independently represents hydrogen, halogen, analkyl group having 1 to 5 carbon atoms or an alkoxy group having 1 to 5carbon atoms, m represents 0, 1 or 2, and n represents 1 to
 4. 8. Themethod of producing a water-based pigment dispersion for ink-jet inkaccording to claim 1, wherein the quinacridone-based pigment is C.I.Pigment Red
 122. 9. An ink composition for ink-jet recording comprising,as a main component, the water-based pigment dispersion for ink-jet inkproduced by the method of any one of claims 1 to 3, 7 and
 8. 10. The inkcomposition for ink-jet recording according to claim 9, which is usedfor a thermal jet type printer.
 11. The method of producing awater-based pigment dispersion for ink-jet ink according to claim 7,wherein the compound represented by the formula (I) is a compoundrepresented by the following formula (III):

wherein m and n each independently represents 0, 1 or
 2. provided that mand n are not simultaneously
 0. 12. The method of producing awater-based pigment dispersion for ink-jet ink according to claim 1,wherein the phthalimidomethylated quinacridone-based compound is acompound represented by the formula (I):

wherein R and R′ each independently represents hydrogen, fluorine,chlorine, bromine, iodine, or an alkyl group having 1 to 5 carbon atoms,m represents 0, 1 or
 2. and n represents 1 to 4.